Does the I2S standard allow both these devices to share the same I2S bus connection?
No, if you even look only briefly into what I2S is, then you'll notice that it's a unidirectional point-to-point link (not really a bus) with no means to distinguish multiple sources.
how would I select which device is enabled as the input?
Use a switch for the total of two or three lines that constitute the signal.
That works as long as both use the same type of signalling, and the receiving end is able to recover from the loss of synchronicity, and you can tolerate the noise that switchover will cause.
If there's different types of I2S configuration, or you can't work with the transient that the switchover produces, then your only solution is to actually pipe your I2S through digital logic (a microcontroller might be able to produce and consume I2S, and then you can have that finish the last sample of a frame from one source cleanly, reconfigure everything, and then start the transmission from the other source; alternatively, you'll need a small FPGA or a CPLD. At which point, really, maybe look into an I2S device with multiple inputs).